The invention relates to a method for identifying a sample in a container and to an associated measuring device.
A method and an apparatus for measuring the complex dielectric constant of a material by evaluating the degree of detuning of an RF resonator, which is introduced by the presence of the material, is known From DE 43 42 505 C1, in which an RF transmitter for transmitting high-frequency electromagnetic fields of selectable frequency into the RF resonator, a receiver for the resonator field, and a measuring circuit connected to the receiver are provided, whereby the amplitude of the received radio-frequency signals can be determined by way of the measuring circuit.
A variety of methods are known for determining the dielectric properties of materials, which typically are indicated as the dielectric constant Fr and the loss angle tan (δ), or as the complex dielectric constant ∈=∈′−i∈″. Such methods are used, for example, for determining the moisture of materials. These methods are based on the high dielectric constant and the large loss factor of the water in the material and are highly important in industry, for example in measuring the moisture in chemicals, foods, tobacco, coffee, and the like.
In the resonance methods for determining moisture by way of microwaves, the material to be analyzed is introduced into a cavity resonator and the detuning of the cavity resonator caused by the presence of the material is measured, the resonance curve being followed and measured by varying the irradiation frequency. The shift in the resonance frequency and the increase in the resonance half-width value, or the change in quality of the resonator, can be used to derive the dielectric constant, and hence the water content of the material, if the material composition and density are known. For this purpose, calibration curves are generally required, which are obtained by prior measurement of respective substances having different known moisture levels. In most known methods, a separate measurement of the material density is also required.
A method for determining the material moisture content with the aid of a cavity resonator is known from DE 40 04 119 A1 in which, for a known material, by way of a defined selection of the field course of the cavity resonator in the region of the sample to be analyzed, the material moisture content and material density can be determined independently of each other, using a calibration curve wherein the resonance frequency determined by following the resonance curve and the half-width value of the resonance line are determined and evaluated. Again, it is necessary to introduce the material to be analyzed into the cavity resonator in the form of a sample.
From the article “Ein Dielektrizitätskonstanten-Messplatz zur Untersuchung optischer Kristalle im Mikrowellenbereich (A dielectric constant measuring station for the analysis of optical crystals in the microwave range)” in: Kristall und Technik, Volume 10, No. 6, 1975, pp. 695-700, by E. Wehrsdorfer et al., a method similar to that described above is known, wherein the dielectric constant of a sample is determined by loading a cavity resonator operated in the microwave range with a sample. The change in resonance frequency and quality caused by the presence of the sample in the cavity resonator is determined by measuring the resonance curve after introducing the sample into the cavity resonator.
The printed East German patent specification 1 38 468 discloses a method for measuring the complex dielectric constant of dielectric plates which are metallized on one side. For the measurement, a dielectric resonator is placed on the non-metallized side of the plate, and the resonator TE011 mode is excited. The metallized side of the plate acts as the end point of the resonance-capable system. The change in resonance frequency compared to the case in which the dielectric resonator is placed on a metal plate can then be used to determine the dielectric constant of the plate material if the thickness is known.
Recently, commercially available moisture sensors allow the sensor to scan the sample within milliseconds. In this process, the product moisture can be continuously detected as a function of the density and weight, with changes in the product temperature being compensated for automatically. These methods are used, for example, in the manufacture of products in the ceramics industry.
The disadvantage of the known methods, which are based on HF technology, is that they are limited to determining the complex dielectric constant of a sample having known dimensions. This means that the geometry and the exact dimensions of the sample must be known in order to determine the real and imaginary parts of the complex dielectric constant based on a measured quality and frequency shift.
It is also disadvantageous that, in order to determine the dielectric constant, a sensor must be brought in contact with the sample, which should be avoided, particularly with hazardous samples, such as acetone or other flammable fluids.